Flat Holm (Welsh: Ynys Hafren) is a Welsh island lying in the Bristol Channel approximately 3.5 miles from Lavernock Point in South Wales

On May 13, 1897, Guglielmo Marconi, assisted by George Kemp, a Cardiff Post Office engineer, transmitted the first wireless signals over water from Lavernock (near Penarth in Wales) to Flat Holm. The message sent in morse code was 'ARE YOU READY.'

Flat Holm is a small, almost circular island, approximately 500 metres in diameter. It rises in a gentle slope from the exposed western rocky shore to more sheltered easterly cliffs, at the top of which stands the prominent lighthouse.

Flat Holm has also been the site of a cholera sanatorium, but is now uninhabited and is known for its gulls and shelducks. Boats sail to the island from Barry. There has been a lighthouse on the island since the 18th century, originally built by William Crispe . Lighthousekeepers lived at the lighthouse until it was automated in 1988.

In 1975, South Glamorgan Country Council leased the island for the next 99 years, and Flat Holm is now designated as a local nature reserve, as stipulated in that lease.

The name Holm or Holme derives from the Scandinavian for river island. The nearby Steep Holm is considered part of England.